Switch arrangement

ABSTRACT

A switch arrangement is disclosed. The arrangement comprises a number of switches arranged on a substrate, each switch having a resiliently deformable conductive dome for making an electrical connection with a corresponding contact on the substrate; and a number of signal lines for connection to a microprocessor for signalling an operation of a switch. The switches are arranged such that operation of any given switch creates a unique electrical connection between two of the signal lines and each of the signal lines is electrically connected to a conductive surface of a respective dome. Also disclosed is a cover for a portable electronic apparatus, having a number of user operable keys. The keys are connectable to a microprocessor in the apparatus via electrical connections made using the conductive domes.

FIELD OF THE INVENTION

This invention relates to an input device. The device may replace orsupplement part of a conventional keypad comprising a plurality of keys.The device finds particular utility in portable apparatus such as mobiletelephones. The device allows for easy customisation of an apparatusincorporating the device.

BACKGROUND OF THE INVENTION

A keypad device, for example as used on a portable telephone, comprisesseveral individual keys. A typical keypad on a portable telephonegenerally comprises keys for the digits 0–9 as well as the specialkeys * and #. Additional menu and functions keys, such as volumecontrol, cursor control and call answer/reject keys are generallyprovided also.

SUMMARY OF THE INVENTION

In order for the microprocessor controlling the telephone to beresponsive to inputs using the keys it is necessary to somehow signal tothe microprocessor that a given key has been pressed or actuated. In thesimplest arrangement, each key would comprise a switch, and each keywould therefore require its own unique connection to the microprocessor.

Such a connection scheme would involve a large number of discreteconnections to the microprocessor, making the number of inputs to themicroprocessor prohibitively large. In order to reduce the size and costof the microprocessor, it is desirable to be able to minimise the numberof discrete connections made to it.

A known solution to this problem is to multiplex the signals before theyare passed to the microprocessor. Multiplexing involves processing thesignals from the keys such that the number of discrete signal lineswhich are required can be reduced,

One form of multiplexing involves assigning each key to an intersectionof a row and column. In this way, a key press can be detected bymonitoring which row and which column are connected at any given time.

Such a configuration is termed a key matrix. The matrix uses a number ofswitches arranged to create a short circuit between any given row andcolumn. The short circuit is detected by continuous monitoring of thesignal lines associated with each row and column.

A suitable form of switch is termed a ‘key dome’. This is shown at FIG.1 a and takes the form of a hollow dome 100 positioned above a contact120 on a surface of a suitable substrate, such as a printed circuitboard (PCB) 110. In its normal rest state, the dome 100 is remote fromthe contact 120 and no connection is made. However, when the dome isdepressed by a user actuating a key, the dome resiliently deforms asshown in FIG. 1 b, and electrical contact is made between the dome andthe contact for the duration of the key press. The dome is usuallymanufactured from a plastics material which has been metalised orotherwise treated on its interior surface to be conductive.

The pressure applied to the key dome is usually applied indirectlythrough the use of a key secured in position above the keydome, andaccessible from the exterior of the apparatus. The key usually bears alegend indicating its function to a user.

Once the pressure on the key is released, the key dome, being resilient,assumes its original configuration, and the electrical connectionbetween the key dome 100 and the contact 120 is broken.

A keypad matrix comprises several such keydomes 100, and a typicalarrangement is shown at FIG. 2. Here is shown a keypad matrix suitablefor use with a regular numeric keypad of a portable telephone. Theletters A, B and C refer to columns of the matrix, and the numbers 1, 2,3 and 4 refer to rows of the matrix.

According to the customary layout of such keys, the key labelled ‘1’ ispositioned above the key dome labelled A1 positioned at the intersectionof Column A and Row 1; the key labelled ‘2’ is positioned above key domeA2; and so on, until the key labelled ‘#’ is positioned above key domeC4.

All the key domes in any given row are electrically connected together.For instance, key domes A1, B1 and C1 are connected together by thetracks on the printed circuit board which are shown runninghorizontally. These tracks are depicted by the solid lines in FIG. 2.

The contacts 120 lying on the surface of the substrate in any givencolumn are electrically connected together. For instance, the contactsat positions A1, A2, A3 and A4 are connected together by trackspartially located on an interior layer of the PCB which are shownrunning vertically. These tracks are depicted by the broken lines inFIG. 2.

In this example, there are 12 individual keys arranged in a matrix of 4rows by 3 columns. It can be appreciated that this arrangement resultsin a saving of five signal lines: rather than there being 12 signallines—one associated with each key—there are now only seven signallines, each associated with a respective row or column.

If the user of the apparatus presses the key labelled ‘5’, key dome B2will be depressed, and electrical connection between column B and row 2will be made. Connections are detected by the processor by sequentiallyapplying a voltage to each of the column signal lines (A, B, C) and thenmonitoring the row signal lines (1, 2, 3, 4) to see whether they exhibita signal at that voltage. If any one of them does exhibit a signal atthat voltage, then a connection has been made between the row and thatcolumn, and hence a key press is detected, and may be decoded asnecessary.

The process is repeated continuously to detect any key-presses, and maybe summarised as:

Apply a voltage to m^(th) row signal line Monitor 1^(st) column signalline Key press? Monitor 2^(nd) column signal line Key press? . . .Monitor n^(th) column signal line Key press? Increment m and repeatprocess.

The non-numeric keys, such as menu and function keys, normally presenton a portable telephone, and previously referred to, may also form partof the matrix, even though they may not appear in a matrix-likeconfiguration to the user i.e. the term ‘matrix’ refers to the manner ofconnections, and not necessarily to the physical appearance of theassociated switches to a user.

Current portable telephones may typically use a keypad matrix having atotal of ten signal lines for connection to the device's microprocessor.That number of signal lines can support a total of up to 25 (5×5) keys.Normally, not all possible connections are used.

Portable telephones are increasingly incorporating functions andfeatures not directly associated with telephony. For instance, it is nowquite usual for portable telephones to come equipped with personalorganiser functions (e.g. the Nokia® 9110 Communicator). Portabletelephones are also available which are equipped with the ability toplay computer games. As the displays of portable telephones increase intheir capabilities e.g. range of colours, size and resolution, moreadvanced games can be offered.

A problem with offering a user of the telephone a gaming experiencecomparable with a dedicated gaming device is the limitation in userinput caused by the configuration of the input keys. The configurationof the keypad is generally dictated by the primary function of theapparatus, which in this case is telephony. Hence, the apparatusrequires a numeric keypad arranged in a manner familiar to the user.This can be incompatible with the requirements of a keypad for gamingpurposes. Further, the tactile response of keys using key domes may notbe appropriate for gaming use, where keys may be operated repetitivelyat relatively high frequencies.

Dedicated handheld gaming devices such as the Nintendo® Gameboy, shownin FIG. 3 offer user input keys in a configuration optimised for gameplay. This configuration is significantly different to the configurationpreferred for telephony functions.

FIG. 3 shows a gaming console 300 having a display 310. Beneath thedisplay are provided several game control keys, including: a directioncontroller 320 comprising four different switches corresponding to fourdifferent directions; two game-specific control keys 330; and two systemcontrol keys 340.

In the field of portable telephones, it is known to provideuser-changeable front covers for telephones to enable a user of thetelephone to customise the appearance of the telephone. Such covers areavailable for several telephones produced by the applicant of thisapplication, such as the 5110 telephone, the 3210 telephone and the 3310telephone.

However, whilst the appearance of the casing may be different, theposition and functions of the individual keys are not user-definable:the position of each individual key is dictated by the fixed position ofits corresponding key dome forming part of the key matrix on anunderlying PCB.

This is illustrated in FIG. 4, which shows an exploded view of a Nokia®5110 portable telephone. The telephone has a body portion 400 whichcontains all the electronic circuitry associated with a portabletelephone.

The user-removable front cover 420 is adapted to fit the body of thetelephone. The front cover 420 may be provided with various surfacedecoration effects such as different colours, textures and patterns.

The front cover 420 has several apertures 430 each corresponding to akey provided on a plastics material keymat 410 which fits between thefront cover 420 and the body 400, when assembled. Each key on the keymat 410 is positioned directly above a corresponding key dome 100disposed on the body of the telephone. Thus, when the telephone isassembled, pressure on a key protruding through an aperture in the frontcover can cause actuation of an associated key dome switch in the bodyof the telephone.

As can be clearly seen from FIG. 4, the position of the keys accessibleto the user via the front cover of the telephone is determined solely bythe position of the corresponding key domes with which the keys mustco-operate in order to function.

According to a first aspect of the present invention, there is provideda switch arrangement comprising: a plurality of switches arranged on asubstrate, each switch comprising a resiliently deformable conductivedome for making an electrical connection with a corresponding contact onthe substrate; and a plurality of signal lines for connection to amicroprocessor for signalling an operation of a switch, wherein theswitches are arranged such that operation of any given switch creates aunique electrical connection between two of the plurality of signallines and each of the plurality of signal lines is electricallyconnected to a conductive surface of a respective dome.

Arranging the switch matrix such that the signal lines are accessiblefrom an exterior surface of the domes enables direct connection to bemade to the microprocessor via the domes themselves, which act asresilient electrical connectors. In this way, new covers havingdifferent arrangements of keys can be added to the apparatus simply, andconnection can be made to the microprocessor without any special stepsneeding to be taken by the user.

According to a second aspect of the present invention, there is provideda cover, for attachment to an electronic apparatus having a plurality ofkey domes disposed in a predetermined pattern, the cover comprising: aplurality of contacts positioned for registration with conductive keydomes of the electronic apparatus; discontinuous electrical pathwaysbetween respective ones of the plurality of contacts; and a plurality ofuser accessible keys arranged such that actuation of respective ones ofthe plurality of keys completes the electrical pathway betweenrespective pairs of the plurality of contacts.

Covers, particularly front covers, according to embodiments of theinvention, have the advantage that the key layout thereon can be madeindependent of the positions of the key domes on the underlyingelectronic apparatus. This allows new covers to be added to the basicapparatus, each new cover being optimised for a particular task. Coversmay be provided which are optimised for game play, text entry or anyother specialised task where a regular numeric key layout is not thepreferred option.

The cover provides a network of incomplete circuits which are completedwhen a given key is operated by a user. In this way, it is possible tosignal to the microprocessor which key has been pressed by gainingdirect access to the microprocessor via the conductive key domes of theapparatus.

According to a third aspect of the present invention, there is provideda portable radio telephone comprising: a body portion having a pluralityof switches, each switch comprising a resiliently deformable conductivedome; and a removable cover attachable to the body portion forco-operation with the domes and comprising a plurality of user operablekeys, wherein the relative positions of the keys on the cover areindependent of the relative positions of the domes.

An advantage of the present invention is that a manufacturer is able tosupply a device, such as a portable telephone, with a default frontcover and key arrangement. The individual key switches may be configuredto work in a similar fashion to prior art key switches by using fingerpressure on a key to deform a key dome and so make an electricalconnection with an underlying contact. Such a default front cover can berelatively inexpensive.

In addition to this default mode of operation, it is possible to supplyfurther changeable front covers having different key arrangementssuitable for specific purposes. The key arrangements of such covers arenot limited by the positions of the key domes arranged on the body ofthe telephone. Such front covers may be made available for use withdifferent applications used on the telephone. An example of such anapplication may be a game. The user may purchase or download a game forhis telephone. As an optional extra, he may be able to purchase a newfront cover which has a key arrangement which has been optimised forgame play.

According to a fourth aspect of the present invention, there is provideda detector for detecting actuation of one of a plurality of switches,each switch being arranged to create a unique electrical connectionbetween two of a plurality of signal lines, wherein the detector isarranged to: apply a known signal to each one of the plurality of signallines in turn; and monitor the remaining ones of the plurality of signallines for the presence of the known signal.

In prior art switch matrices, a key press is indicated to amicroprocessor by the creation of an electrical connection between anysingle row and any single column. Therefore, for a matrix having m rowsand n columns, a total of m×n switches may be supported.

According to embodiments of the invention, the microprocessor of thetelephone is capable of detecting and interpreting an electricalconnection between any two signal lines regardless of whether they areboth column signal lines or row signal lines. By providing a front coverwith an arrangement of switches whereby an electrical connection may becreated between any two signal lines, it is possible to increase thenumber of keys which may be supported by a microprocessor with nocorresponding increase in the number of signal lines required.

This result is a direct consequence that the number of connectionsbetween different signal lines is limited in the case where a simple keydome matrix configuration is used. As shown, the maximum number ofswitches possible when there are n+m signal lines is n×m.

However, when the connections are not so restricted, the number ofswitches which may be supported is given by the well known formula forcalculating combinations: nCr, where in this case, n=number of signallines and r=2 (as two signal lines must be connected to establish a keypress).

The mathematical formula for nCr is:

$\begin{matrix}{{n\; C\; r} = \frac{n!}{{( {n - r} )!}{r!}}} & (1)\end{matrix}$

For any given number of signal lines it is possible to support moreswitches if connection is possible between any two of them, than ifconnection is only possible between individual members of two definedsubsets (e.g. rows and columns) of signal lines.

In prior art devices using a matrix switch arrangement, the connectionsare achieved by a physical short circuit between a key dome and anunderlying contact i.e. the connection is made physically within theswitch arrangement.

Embodiments of the invention allow such a connection to be made somewhatremote from the actual switch matrix. In some embodiments described, thephysical connections are made in a removable front cover i.e. theconnections are made distant from the switch matrix, but communicated tothe microprocessor through the medium of the key domes which form partof the switch matrix.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, and to understandhow the same may be brought into effect, the invention will now bedescribed, by way of example only, with reference to the appendeddrawings in which:

FIGS. 1 a & 1 b show a cross sectional view of a prior art key domeswitch before and after actuation;

FIG. 2 shows a prior art arrangement of a key switch matrix comprising aplurality of key dome switches;

FIG. 3 shows a prior art handheld gaming console;

FIG. 4 shows an exploded view of the assembly of a prior art portabletelephone having an exchangeable front cover;

FIG. 5 shows a key switch matrix and associated connections according toan embodiment of the invention;

FIG. 6 shows the possible interconnections of the switch arrangement ofFIG. 5;

FIG. 7 shows a partial cross-section through a portable telephone havinga removable front cover, all according to embodiments of the invention;and

FIG. 8 shows all possible switch arrangements according to an embodimentof the invention.

DETAILED DESCRIPTION OF THE INVENTION

A key switch matrix according an embodiment of the invention is shown inFIG. 5. The matrix has three columns and 4 rows, and thus is able toaccommodate 12 (3×4) switches as shown. The basic mode of operation isas previously described with respect to the prior art matrix of FIG. 2i.e. depressing a key dome 500 causes it to resiliently deform and somake a connection with the corresponding contact 510 on the surface of aPCB. This connection momentarily connects the row and column at whoseintersection the switch lies. This interconnection may be detected andinterpreted as previously described in order to react to a key press.

Unlike the prior art key domes, the key domes of embodiments of theinvention are manufactured from a metallic material, or from a plasticsmaterial treated to be conductive on interior and exterior surfaces andfor the surfaces to be electrically connected together.

A difference between the matrix of FIG. 5 and prior art matrices is themanner in which the rows and columns are configured, and which elementsof the switch (key domes or contacts) are permanently interconnected.

In the embodiment shown in FIG. 5, it can be seen that the connectionsbetween key domes 500 and contacts 510 are significantly different tothe connections shown in the prior art matrix of FIG. 2. In the priorart matrix, all the key domes in a given row are mutuallyinterconnected, and all the contacts in a given column are mutuallyinterconnected.

In FIG. 5, the interconnections for the various rows and columns are:

-   Row 1: KD-A; KD-B; Con-C.-   Row 2: KD-A; KD-B; KD-C.-   Row 3: KD-A; KD-B; KD-C.-   Row 4: Con-A; Con-B; KD-C-   Column A: Con-1; Con-2; Con-3; KD-4.-   Column B: Con-1; Con-2; Con-3; KD-4.-   Column C: KD-1; Con-2; Con-3; Con-4.

The nomenclature used above means that e.g. the permanentinterconnections in Row 1 are between the key dome of Column A, the keydome of column B and the contact of column C. The same nomenclature isused for all other rows and columns.

The solid interconnect lines of FIG. 5 represent tracks on the surfaceof the PCB, and the broken lines represent tracks which are routed on aninterior layer of the PCB. Tracks need to be routed on interior layersso that unwanted permanent short circuits do not occur between certainsignal lines.

Using this configuration of connections, the same number of signal linesas in the prior art solution are required, hence the same economies ofconnections to the processor may be achieved. However, embodimentsaccording to the invention can facilitate a number of externalconnections to the processor. Such connections are possible by makingelectrical contact to the exposed surface of the conductive key domes.In prior art solutions, the exterior surface of the key domes aregenerally not conductive, or even if they are, not all signal linescould be accessed in this way as some of the signal lines are onlyconnected to the contacts on the substrate, rendering them inaccessiblefor external connection.

Embodiments of the invention allow all keypad connections to be madeexternally accessible for other uses. An example of another use is theaddition of a new front cover to a mobile telephone. Unlike thechangeable front cover of the prior art shown at FIG. 4, a changeablefront cover according to embodiments of the invention may make use ofthe available connections to the keypad matrix connections via theexposed surfaces of the key domes to allow a different configuration ofkeys to be adopted on the new front cover. The arrangement of the keysis not limited by the positions of the key domes on the telephone in theway that prior art changeable front covers are. In this way, new coversmay be added which are specifically designed for particular functions byarranging keys in optimum positions, and having defined tactileresponses.

An example of such a cover may be one which is optimised for gaming use,and may resemble the key layout of the games console shown in FIG. 3.Such a cover may have keys having characteristics which are optimisedfor gaming purposes. For instance, the tactile response may be designedin a certain way for particular games types.

The configuration of connections shown in FIG. 5 is exemplary only, andother configurations are possible which also achieve the desired effectof making every signal line available via a conductive surface of a keydome.

Additionally, embodiments of the invention allow for more switches to besupported on a changeable front cover. Using the simple matrix switchingarrangement of the prior art and relying on deformation of the keydomes, it is only possible to create an electrical connection betweenany row signal line and any column signal line. However, themicroprocessor is capable of detecting an electrical connection betweenany two signal lines, regardless of their designation as row or column.

A front cover according to embodiments of the invention can utilise amore complex switching arrangement than the prior art solution, as it isnot limited to making connections only between respective row and columnsignal lines.

This is illustrated in FIG. 6. The first box 550 shows the twelveconnections possible when the switch arrangement of FIG. 5 is operatingin a prior art mode, i.e. connections are detected on the basis of a keydome being resiliently deformed and contacting an underlying contact.The vertical lines indicate the connections which are possible betweenthe row signal lines (1, 2, 3, 4) and the column signal lines (A, B, C).

When the switch arrangement of FIG. 5 is operating in a mode accordingto embodiments of the invention with a front cover arranged accordingly,additional switch combinations may be supported. These extracombinations of connection are shown in the second box 560 of FIG. 6.These connections are in addition to the ones shown in the first box,which are also still possible. Here again, the vertical lines indicatepossible connections. Note that it is now no longer only a row signalline which can connect to a column signal line. Any signal line mayconnect to any other.

In the example shown, with a prior art arrangement of a 4×3 matrix,seven signal lines can support twelve switches. However embodimentsaccording to the invention can support twenty one switches using thesame seven signal lines. The figure of 21 is derived from formula (1)given previously, with n=7 and r=2.

Unlike the detection method described in relation to the prior artmatrix where each column signal line is energised in turn, and the rowsignal lines are monitored sequentially to determine whether aconnection has been made as the result of a key press, embodiments ofthe invention operate to energise each signal line (A, B, C, 1, 2, 3, 4)in turn, and monitor each of the remaining signal lines in turn todetermine whether a connection has been made.

For example, the process may be:

Signal Line A has a voltage applied to it Signal line B is monitored Keypress? Signal line C is monitored Key press? Signal line 1 is monitoredKey press? Signal line 2 is monitored Key press? Signal line 3 ismonitored Key press? Signal line 4 is monitored Key press? Signal Line Bhas a voltage applied to it Signal line A is monitored Key press? Signalline C is monitored Key press? Etc.

In this way, with a simple modification to the microprocessor, it ispossible to detect a connection between any two signal lines regardlessof whether they are designated as row or column lines. This detectionprocess operates with any type of front cover regardless of whether itis operating in prior art mode relying on depression of the individualkey dome switches, or whether it is operating according to embodimentsof the invention, in which the key dome switches are not depressed, butrather used as contacts to the signal lines.

As a consequence of being able to support more keys on a front coveraccording to embodiments of the invention, it is possible to offer afront cover which has a full qwerty-style keyboard without needing toincrease the number of signal lines required to connect to themicroprocessor.

A typical number of signal lines is ten. Using formula (1), with 10signal lines (n=10), it is possible to support 45 different switches.This is more than sufficient to support the number of keys required fora full alphanumeric keyboard.

FIG. 7 shows a cross section through a front cover attached to atelephone body according to an embodiment of the invention. The frontcover is user-removable and may be configured for attachment in a‘snap-on’ fashion, or alternatively, the user may be required to use atool to attach/detach the front cover. Details of a suitable means ofattachment is disclosed in co-assigned patent application EP0883955A.

Unlike the prior art front cover of FIG. 4, the cover of FIG. 7 may beconfigured to have an arrangement of keys different to the arrangementof key domes on the surface of the telephone to which it is attached.This is because the actuation of a key on the front cover does not relyon the resilient deformation of a key dome in order to make a contactand signal to the telephone's microprocessor that a key has beenpressed. Therefore, there is no need for the key on the front cover tobe positioned directly above any particular key dome on the body of thetelephone.

Instead, a key press is signalled to the microprocessor by makingconnections directly between the signal lines connected to themicroprocessor. This is possible due to the way in which the key domeshave been configured so that each signal line is directly connected to aconductive dome of the key dome arrangement. In effect, the key domesact as electrical contacts for allowing direct connection to themicroprocessor. This is in contrast to prior art solutions where the keydomes only operated as switches, and not as herein described. Prior artdevices can not operate according to embodiments of the invention as thesignal line connections were not externally accessible in this way.

The front cover according to embodiments of the invention incorporates akeypad matrix circuit which is distinct from the matrix circuit of thetelephone. This operates in a similar manner to the matrix circuit ofthe telephone, but may use different switch types. The operation of thefront cover matrix circuit is described later in relation to FIG. 8.Essentially, each key of the front cover corresponds to a switchintegrated into the front cover. The switches are arranged so thatactuation of any given switch results in an electrical connectionbetween two signal lines.

The electrical connection between two signal lines is mapped to anelectrical connection between two of the signal lines (A, B, C, 1, 2, 3,4) connected to the microprocessor.

FIG. 7 shows a cross section through a front cover attached to atelephone. The parts 600 and 610 form part of the body of the telephone.The parts 620–680 form part of the removable front cover.

The front surface of the telephone body 600, which is exposed when nocover is present, is a substrate such as a printed circuit board (PCB)on which several key domes 610 are disposed.

Attached to the telephone body is a front cover having a key arrangementwhich is not dictated by the positions of the key domes 610 on thetelephone. The front cover has an exposed surface 620, which may becoloured or patterned as required. The front surface 620 has severalapertures through which are exposed user-operable keys 630. The keys maybe formed individually or as a key-mat from a silicone rubber or similarmaterial. The exposed surface of the key may carry a legend indicatingto a user the function of the key. The rear surface 640 of the key has aconductive element attached or integrally formed. An example of asuitable element may be a so-called ‘carbon pill’. This is disc likeelement composed of conductive carbon.

When a key 630 is pressed, the pill 640 is brought into contact withconductive tracking 650 on the upper surface of PCB 660. The PCB may bemanufactured using known methods, and the tracking may be etched copperor printed silver or any other suitable material.

The effect of the pill 640 contacting the tracking 650 is to create anelectrical connection between two previously unconnected tracks. Theeffect produced is similar to depressing a key dome switch in the priorart discussed previously in that the connection formed is made betweentwo signal lines and is a unique indication corresponding to aparticular key 630. However, the connections are not limited to beingbetween a row and column signal line.

FIG. 8 shows a possible switch arrangement for use in a front coverconnectable to a portable telephone having seven signal lines connectedto a microprocessor. Each signal line (A, B, C, 1, 2, 3, 4) is connectedto an exterior conductive surface of a respective key dome as previouslydescribed. Each signal line is further connected via the resilientconnector 680 to tracking 650 on a PCB 660 in the front cover.

The resilient connector 680 is shaped and configured to apply enoughforce to the key dome 610 to ensure a reliable electrical connection,but not so much force that the key dome is deformed and makes aconnection with its underlying contact. Using typically sized andmanufactured parts, a force equivalent to a mass of 150 g is needed todeform a key dome sufficiently to make contact according to prior arttechniques. A force equivalent to a mass of 80 g is sufficient to ensurea reliable connection between the spring contact 680 and key dome 610without causing the dome to deform.

Unlike the prior art solution, the tracking can be arranged such thattwo exposed contacts lie on PCB 660 immediately beneath key 630. Whenkey 630 is depressed, the conductive rear 640 of the key makes anelectrical connection between the two contacts, and thus forms aconnection between two signal lines.

FIG. 8 shows all 21 connections which are possible using seven signallines. In the scheme of FIG. 8: signal line A is connectable to B, C, 1,2, 3 and 4; signal line B is connectable to A, C, 1, 2, 3 and 4, and soon for the other signal lines.

To further enhance the operation of the front cover, it incorporates alight guide 670 which is at least partially transparent and allows lightto pass through from light sources on the body of the telephone (notshown) to illuminate the keys 630 of the front cover. For the samereason, the PCB 660 may be constructed from a material which allowslight to pass through. The tracking 650 occupies a relatively small areaof the PCB and so does not interfere with the passage of the light to agreat extent.

The present invention includes any novel feature or combination offeatures disclosed herein either explicitly or any generalisationthereof irrespective of whether or not it relates to the claimedinvention or mitigates any or all of the problems addressed.

1. A cover, for attachment to an electronic apparatus having a pluralityof key domes disposed in a predetermined pattern, the cover comprising:plurality of contacts positioned for registration with conductive keydomes of the electronic apparatus; discontinuous electrical pathwaysbetween respective ones of the plurality of contacts; and a plurality ofuser accessible keys arranged such that actuation of respective ones ofthe plurality of keys completes the electrical pathway betweenrespective pairs of the plurality of contacts.
 2. A cover as claimed inclaim 1 wherein each of the plurality of contacts is resilient and isarranged such that when the cover is attached to the electronicapparatus, the contact provides enough contact force to provide anelectrical connection but not so much force that the key dome issubstantially deformed.
 3. A cover as claimed in claim 1 wherein theposition of an individual key is independent of the position of any ofone of the plurality of key domes of the electronic apparatus.
 4. Acover as claimed in claim 1 wherein the cover comprises more keys thanthe electronic apparatus has key domes.
 5. A cover as claimed in claim 1wherein the keys are arranged for controlling an electronic game.
 6. Acover as claimed in claim 1 wherein a unique key is provided for eachletter of the alphabet.
 7. A cover as claimed in claim 6 wherein thekeys are arranged as a qwerty-style keyboard.
 8. A cover as claimed inclaim 1 wherein the cover is arranged to be removable by a user of theelectronic apparatus.
 9. A cover as claimed in claim 1 wherein the coveris a front cover.
 10. A cover as claimed in claim 1 comprising a lightguide for allowing light to pass through the cover from a surface of theelectronic apparatus.
 11. A cover as claimed in claim 1 wherein anelectrical pathway is completed by a conductive part of a key shortconnecting two discontinuous electrical pathways.
 12. A portableelectronic apparatus comprising a body portion having a switcharrangement comprising: a plurality of switches arranged on a substrate,each switch comprising a resiliently deformable conductive dome formaking an electrical connection with a corresponding contact on thesubstrate; and a plurality of signal lines for connection to amicroprocessor for signaling an operation of a switch, wherein theswitches are arranged such that operation of any given switch creates aunique electrical connection between two of the plurality of signallines and each of the plurality of signal lines is electricallyconnected to a conductive surface of a respective dome and a cover asclaimed in claim
 1. 13. A portable radio telephone comprising: a bodyportion having a plurality of switches, each switch comprising aresiliently deformable conductive dome; and a removable cover comprisingmeans for attachment to the body portion and for co-operation with thedomes and comprising a plurality of user operable keys, wherein therelative positions of the keys on the cover are independent of therelative positions of the domes.
 14. A removable cover for attachment tothe portable radio telephone as claimed in claim 13.